The present invention relates to a process for lowering nitrogen oxide levels in combustion engine exhaust gas by catalytic reduction on a reduction catalyst using the hydrocarbons and carbon monoxide also present in the exhaust gas as reducing agents to yield nitrogen, water and carbon dioxide.
During normal operation, the exhaust gas from diesel engines and Otto cycle engines operated with a lean mixture (known as lean-burn engines) contains a high proportion of 3 to 10 vol. % of oxygen in addition to uncombusted hydrocarbons, carbon monoxide and nitrogen oxides. Due to the super stoichiometric oxygen content of the exhaust gas, it is not possible to convert all three pollutants simultaneously using the three-way method conventional for Otto cycle engines. Otto cycle engines, also known as gasoline internal combustion engine, are usually operated with air ratios, .lambda., of around 1, while diesel engines and lean-burn engines work at air ratios of approximately 1.2 and above. The air ratio, .lambda., is the air/fuel ratio standardized for stoichiometric operation (kilogram air/kilogram fuel).
The uncombusted hydrocarbons and carbon monoxide in diesel exhaust and the exhaust from lean-burn engines may relatively readily be converted by oxidation catalysts. In contrast, special reduction catalysts must be used to convert the nitrogen oxides. Such catalysts are described, for example, in "Design Aspects of Lean NO.sub.x -Catalysts for Gasoline and Diesel Engine Applications" by Leyrer et al. in SAE-Paper no. 95 2485, 1995 and in "Catalytic reduction of NO.sub.x with hydrocarbons under lean diesel exhaust gas conditions" by Engler et al. in SAE-Paper no. 930735, 1993. Zeolite-based catalysts, which may be exchanged with various catalytically active metals (for example copper or platinum), are used.
These so-called DENOX catalysts reduce the nitrogen oxides while simultaneously oxidizing hydrocarbons and carbon monoxide. The conversion rates for the individual pollutant components are highly dependent upon exhaust gas temperature. As exhaust gas temperature rises, oxidation of the hydrocarbon and carbon monoxide begins first and, within a temperature range from 150 to 175.degree. C., reaches oxidation rates of above 90%. As temperature rises further, hydrocarbon conversion remains constant. The exhaust gas temperature at which a conversion rate of 50% is achieved for a particular pollutant is known as the light off temperature for this pollutant.
The conversion rate for nitrogen oxides follows the hydrocarbon conversion rate. However, it does not rise uniformly, instead passing through a maximum at temperatures at which hydrocarbon oxidation has virtually achieved its maximum value and then falling back virtually to zero as temperature increases further. Optimum conversion rates for nitrogen oxides are thus achieved only within a narrow temperature window.
The conversion curves for the individual pollutants are highly dependent upon the formulation of the particular catalyst. This also applies to nitrogen oxides: the position and size of the temperature window and the maximum conversion rate achievable within this window are dependent upon the catalyst formulation. So-called low-temperature catalysts are known which achieve maximum nitrogen oxide conversion at temperatures of between 200 and 250.degree. C. In the case of high-temperature catalysts, maximum nitrogen oxide conversion is above 300.degree. C.
The great temperature dependence of nitrogen oxide conversion rates is a major problem for the purification of diesel exhaust gases as the engine outlet temperature of the exhaust gases in operating diesel vehicles may vary between approximately 150 and 500.degree. C. depending upon driving conditions. Elevated conversion rates are thus achieved only during short phases of operation during which the exhaust gas temperature is within the optimum range for the catalyst used.
A further problem in this connection is also the trend in modern catalyst development to develop catalysts with ever lower light off temperatures. In these catalysts, the window for maximum conversion of nitrogen oxides is, of course, also shifted towards lower temperatures, such that virtually no nitrogen oxides are converted at higher exhaust gas temperatures.
In order to ensure nitrogen oxide conversion over a wider temperature range, it has already been attempted to combine low-temperature and high-temperature catalysts or to inject additional hydrocarbons as a reducing agent into the exhaust gas stream shortly upstream from the catalyst.
An object of the present invention is to avoid the shortcomings and drawbacks of prior known methods of exhaust gas purification.
A further object of the present invention is to attain elevated conversion of the nitrogen oxides even at temperatures above the maximum nitrogen oxide conversion of the catalyst used.